1. Field of the Invention
The present invention relates generally to semiconductor integrated circuit (IC) diagnostics, and, in particular, to methods of non-invasive optical IC diagnostics using light emission from leakage currents in field-effect transistors and MOS capacitors.
2. Background Art
The progress of microelectronics complicates IC diagnostics, making many existing methods obsolete. For example, electrical IC characterization using mechanical probes is difficult and will become more difficult due to shrinking transistor sizes, multiple metal layers, and increasing IC complexity.
Mechanical probing faces the following major problems. First, the access to deep submicron wires requires complex and expensive techniques like Focused Ion Beam (FIB) for probe point creation. More important, the increasing number of metal layers often makes impossible the access to wires deep in the metal stack. Second, the load capacitance and/or resistance of mechanical probes limits the bandwidth of timing measurements.
Third, the proliferation of flip-chip IC packaging makes the access to the front side of an IC impossible, and, consequently, renders this type of probing obsolete.
Another example of electrical IC characterization is e-beam probing. This method provides better spatial and timing resolution while being significantly more expensive. It suffers basically from the same problems as the previous technique: the increasing number of metal layers and the front side inaccessibility.
Photon emission microscopy (PEM) is an alternative method of IC diagnostics. This method uses visible and near infrared photon emission from transistors, pn junctions and similar structures to make conclusions about the operation of an IC. PEM can use time integrating detectors (such as CCD cameras, Focal Plane Arrays, etc.) to obtain the data, as well as time-resolved detectors (such as multichannel plate photomultipliers, single photon avalanche diodes, photomultipliers, etc.). The PEM can be performed both from the front-side and back-side of an IC.
The back-side PEM enjoys increasing interest due to the fact that it circumvents the two major problems of other techniques: multiple metal layers and flip-chip packaging.
Until recently, the only source of light emission from MOSFETs was hot electron radiation from saturated devices. All existing PEM techniques for MOSFETs are based on this type of light emission. As the size of MOSFETs decreases, two types of parasitic leakage currents become increasingly important—gate tunneling current (this type of current is also present in MOS capacitors) and OFF-state drain to source current. Each type of leakage current results in photon emission or, simply, leakage light. In general, the leakage light increases with the increase of leakage current as well as the increase of the voltage difference applied to the device. The leakage light also depends on the device temperature, it increases with the increase of temperature.
The analysis of leakage light may provide valuable insights into optical IC diagnostics. There is, therefore, a need in the art of PEM analysis of ICs for methods to extract information about IC operation from leakage current emission measurements.